The search for patterns in data sets is a fundamental task in Data Mining, where Machine Learning algorithms are generally used.
However, Machine Learning algorithms have biases that strengthen the classification task, not taking into consideration exceptions. Exceptions contradict common sense rules. They are generally unknown, unexpected and contradictory to the user believes. For this reason, exceptions may be interesting. In this work we propose a method to find exceptions out from common sense rules. Besides, we apply the proposed method in a real world data set, to discover rules and exceptions in the HIV virus protein cleavage process.